Method and apparatus for handling sidelink radio link failure in a wireless communication system

ABSTRACT

A method and device are disclosed from the perspective of a User Equipment-to-User Equipment (UE-to-UE) Relay. In one embodiment, the method includes the UE-to-UE Relay establishing a first PC5 unicast link with a first UE and establishes a second PC5 unicast link with a second UE. The method further includes the UE-to-UE Relay initiating a Layer-2 link release procedure to release the second PC5 unicast link if a sidelink radio link failure is detected on the first PC5 unicast link.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/045,744 filed on Jun. 29, 2020, the entire disclosure of which is incorporated herein in its entirety by reference.

FIELD

This disclosure generally relates to wireless communication networks, and more particularly, to a method and apparatus for handling sidelink radio link failure in a wireless communication system.

BACKGROUND

With the rapid rise in demand for communication of large amounts of data to and from mobile communication devices, traditional mobile voice communication networks are evolving into networks that communicate with Internet Protocol (IP) data packets. Such IP data packet communication can provide users of mobile communication devices with voice over IP, multimedia, multicast and on-demand communication services.

An exemplary network structure is an Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The E-UTRAN system can provide high data throughput in order to realize the above-noted voice over IP and multimedia services. A new radio technology for the next generation (e.g., 5G) is currently being discussed by the 3GPP standards organization. Accordingly, changes to the current body of 3GPP standard are currently being submitted and considered to evolve and finalize the 3GPP standard.

SUMMARY

A method and device are disclosed from the perspective of a User Equipment-to-User Equipment (UE-to-UE) Relay. In one embodiment, the method includes the UE-to-UE Relay establishing a first PC5 unicast link with a first UE and establishes a second PC5 unicast link with a second UE. The method further includes the UE-to-UE Relay initiating a Layer-2 link release procedure to release the second PC5 unicast link if a sidelink radio link failure is detected on the first PC5 unicast link.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a wireless communication system according to one exemplary embodiment.

FIG. 2 is a block diagram of a transmitter system (also known as access network) and a receiver system (also known as user equipment or UE) according to one exemplary embodiment.

FIG. 3 is a functional block diagram of a communication system according to one exemplary embodiment.

FIG. 4 is a functional block diagram of the program code of FIG. 3 according to one exemplary embodiment.

FIG. 5 is a reproduction of FIG. 5.2.1.4-1 of 3GPP TS 23.287 V16.2.0.

FIG. 6 is a reproduction of FIG. 6.1.1-1 of 3GPP TS 23.287 V16.2.0.

FIG. 7 is a reproduction of FIG. 6.1.2-1 of 3GPP TS 23.287 V16.2.0.

FIG. 8 is a reproduction of FIG. 6.3.3.1-1 of 3GPP TS 23.287 V16.2.0.

FIG. 9 is a reproduction of FIG. 6.3.3.3-1 of 3GPP TS 23.287 V16.2.0.

FIG. 10 is a reproduction of FIG. 6.3.3.4-1 of 3GPP TS 23.287 V16.2.0.

FIG. 11 is a reproduction of FIG. 6.3.3.5-1 of 3GPP TS 23.287 V16.2.0.

FIG. 12 is a reproduction of FIG. 6.1.2.2.2 of 3GPP TS 24.587 V16.0.0.

FIG. 13 is a reproduction of FIG. 5.8.9.1.1-1 of 3GPP TS 38.331 V16.0.0.

FIG. 14 is a reproduction of FIG. 5.8.9.1.1-2 of 3GPP TS 38.331 V16.0.0.

FIG. 15 is a reproduction of FIG. 6.8.2-1 of 3GPP TR 23.752 V0.3.0.

FIG. 16 is a reproduction of FIG. 6.9.2-1 of 3GPP TR 23.752 V0.3.0.

FIG. 17 is a reproduction of FIG. 6.10.2-1 of 3GPP TR 23.752 V0.3.0.

FIG. 18 illustrates an exemplary integrated PC5 unicast link via a UE-to-UE Relay according to one embodiment.

FIG. 19 illustrates exemplary relay UE behaviors in response to detection of radio link failure on one unicast link according to one embodiment.

FIG. 20 is a flow chart according to one exemplary embodiment.

DETAILED DESCRIPTION

The exemplary wireless communication systems and devices described below employ a wireless communication system, supporting a broadcast service. Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), 3GPP LTE (Long Term Evolution) wireless access, 3GPP LTE-A or LTE-Advanced (Long Term Evolution Advanced), 3GPP2 UMB (Ultra Mobile Broadband), WiMax, 3GPP NR (New Radio), or some other modulation techniques.

In particular, the exemplary wireless communication systems and devices described below may be designed to support one or more standards such as the standard offered by a consortium named “3rd Generation Partnership Project” referred to herein as 3GPP, including: TS 23.287 V16.2.0, “Architecture enhancements for 5G System (5GS) to support Vehicle-to-Everything (V2X) services (Release 16)”; TS 24.587 V16.0.0, “Vehicle-to-Everything (V2X) services in 5G System (5GS); Stage 3 (Release 16)”; TS 38.331 V16.0.0, “NR; Radio Resource Control (RRC) protocol specification (Release 16)”; and TR 23.752 V0.3.0, “Study on system enhancement for Proximity based services (ProSe) in the 5G System (5GS) (Release 17)”. The standards and documents listed above are hereby expressly incorporated by reference in their entirety.

FIG. 1 shows a multiple access wireless communication system according to one embodiment of the invention. An access network 100 (AN) includes multiple antenna groups, one including 104 and 106, another including 108 and 110, and an additional including 112 and 114. In FIG. 1, only two antennas are shown for each antenna group, however, more or fewer antennas may be utilized for each antenna group. Access terminal 116 (AT) is in communication with antennas 112 and 114, where antennas 112 and 114 transmit information to access terminal 116 over forward link 120 and receive information from access terminal 116 over reverse link 118. Access terminal (AT) 122 is in communication with antennas 106 and 108, where antennas 106 and 108 transmit information to access terminal (AT) 122 over forward link 126 and receive information from access terminal (AT) 122 over reverse link 124. In a FDD system, communication links 118, 120, 124 and 126 may use different frequency for communication. For example, forward link 120 may use a different frequency then that used by reverse link 118.

Each group of antennas and/or the area in which they are designed to communicate is often referred to as a sector of the access network. In the embodiment, antenna groups each are designed to communicate to access terminals in a sector of the areas covered by access network 100.

In communication over forward links 120 and 126, the transmitting antennas of access network 100 may utilize beamforming in order to improve the signal-to-noise ratio of forward links for the different access terminals 116 and 122. Also, an access network using beamforming to transmit to access terminals scattered randomly through its coverage causes less interference to access terminals in neighboring cells than an access network transmitting through a single antenna to all its access terminals.

An access network (AN) may be a fixed station or base station used for communicating with the terminals and may also be referred to as an access point, a Node B, a base station, an enhanced base station, an evolved Node B (eNB), a network node, a network, or some other terminology. An access terminal (AT) may also be called user equipment (UE), a wireless communication device, terminal, access terminal or some other terminology.

FIG. 2 is a simplified block diagram of an embodiment of a transmitter system 210 (also known as the access network) and a receiver system 250 (also known as access terminal (AT) or user equipment (UE)) in a MIMO system 200. At the transmitter system 210, traffic data for a number of data streams is provided from a data source 212 to a transmit (TX) data processor 214.

In one embodiment, each data stream is transmitted over a respective transmit antenna. TX data processor 214 formats, codes, and interleaves the traffic data for each data stream based on a particular coding scheme selected for that data stream to provide coded data.

The coded data for each data stream may be multiplexed with pilot data using OFDM techniques. The pilot data is typically a known data pattern that is processed in a known manner and may be used at the receiver system to estimate the channel response. The multiplexed pilot and coded data for each data stream is then modulated (i.e., symbol mapped) based on a particular modulation scheme (e.g., BPSK, QPSK, M-PSK, or M-QAM) selected for that data stream to provide modulation symbols. The data rate, coding, and modulation for each data stream may be determined by instructions performed by processor 230.

The modulation symbols for all data streams are then provided to a TX MIMO processor 220, which may further process the modulation symbols (e.g., for OFDM). TX MIMO processor 220 then provides N_(T) modulation symbol streams to N_(T) transmitters (TMTR) 222 a through 222 t. In certain embodiments, TX MIMO processor 220 applies beamforming weights to the symbols of the data streams and to the antenna from which the symbol is being transmitted.

Each transmitter 222 receives and processes a respective symbol stream to provide one or more analog signals, and further conditions (e.g., amplifies, filters, and upconverts) the analog signals to provide a modulated signal suitable for transmission over the MIMO channel. N_(T) modulated signals from transmitters 222 a through 222 t are then transmitted from N_(T) antennas 224 a through 224 t, respectively.

At receiver system 250, the transmitted modulated signals are received by N_(R) antennas 252 a through 252 r and the received signal from each antenna 252 is provided to a respective receiver (RCVR) 254 a through 254 r. Each receiver 254 conditions (e.g., filters, amplifies, and downconverts) a respective received signal, digitizes the conditioned signal to provide samples, and further processes the samples to provide a corresponding “received” symbol stream.

An RX data processor 260 then receives and processes the N_(R) received symbol streams from N_(R) receivers 254 based on a particular receiver processing technique to provide N_(T) “detected” symbol streams. The RX data processor 260 then demodulates, deinterleaves, and decodes each detected symbol stream to recover the traffic data for the data stream. The processing by RX data processor 260 is complementary to that performed by TX MIMO processor 220 and TX data processor 214 at transmitter system 210.

A processor 270 periodically determines which pre-coding matrix to use (discussed below). Processor 270 formulates a reverse link message comprising a matrix index portion and a rank value portion.

The reverse link message may comprise various types of information regarding the communication link and/or the received data stream. The reverse link message is then processed by a TX data processor 238, which also receives traffic data for a number of data streams from a data source 236, modulated by a modulator 280, conditioned by transmitters 254 a through 254 r, and transmitted back to transmitter system 210.

At transmitter system 210, the modulated signals from receiver system 250 are received by antennas 224, conditioned by receivers 222, demodulated by a demodulator 240, and processed by a RX data processor 242 to extract the reserve link message transmitted by the receiver system 250. Processor 230 then determines which pre-coding matrix to use for determining the beamforming weights then processes the extracted message.

Turning to FIG. 3, this figure shows an alternative simplified functional block diagram of a communication device according to one embodiment of the invention. As shown in FIG. 3, the communication device 300 in a wireless communication system can be utilized for realizing the UEs (or ATs) 116 and 122 in FIG. 1 or the base station (or AN) 100 in FIG. 1, and the wireless communications system is preferably the NR system. The communication device 300 may include an input device 302, an output device 304, a control circuit 306, a central processing unit (CPU) 308, a memory 310, a program code 312, and a transceiver 314. The control circuit 306 executes the program code 312 in the memory 310 through the CPU 308, thereby controlling an operation of the communications device 300. The communications device 300 can receive signals input by a user through the input device 302, such as a keyboard or keypad, and can output images and sounds through the output device 304, such as a monitor or speakers. The transceiver 314 is used to receive and transmit wireless signals, delivering received signals to the control circuit 306, and outputting signals generated by the control circuit 306 wirelessly. The communication device 300 in a wireless communication system can also be utilized for realizing the AN 100 in FIG. 1.

FIG. 4 is a simplified block diagram of the program code 312 shown in FIG. 3 in accordance with one embodiment of the invention. In this embodiment, the program code 312 includes an application layer 400, a Layer 3 portion 402, and a Layer 2 portion 404, and is coupled to a Layer 1 portion 406. The Layer 3 portion 402 generally performs radio resource control. The Layer 2 portion 404 generally performs link control. The Layer 1 portion 406 generally performs physical connections.

3GPP TS 23.287 specifies procedures related to unicast mode V2X communication over PC5 reference point as follows:

5.1.2 Authorization and Provisioning for V2X Communications Over PC5 Reference Point 5.1.2.1 Policy/Parameter Provisioning

The following sets of information for V2X communications over PC5 reference point is provisioned to the UE:

1) Authorization policy:

-   -   When the UE is “served by E-UTRA” or “served by NR”:         -   PLMNs in which the UE is authorized to perform V2X             communications over PC5 reference point when “served by             E-UTRA” or “served by NR”.         -   For each above PLMN:             -   RAT(s) over which the UE is authorized to perform V2X                 communications over PC5 reference point.     -   When the UE is “not served by E-UTRA” and “not served by NR”:         -   Indicates whether the UE is authorized to perform V2X             communications over PC5 reference point when “not served by             E-UTRA” and “not served by NR”.         -   RAT(s) over which the UE is authorized to perform V2X             communications over PC5 reference point.     -   NOTE 1: In this specification, {When the UE is “served by         E-UTRA” or “served by NR”} and {When the UE is “not served by         E-UTRA” and “not served by NR”} are relevant to V2X         communications over PC5 reference point.

2) Radio parameters when the UE is “not served by E-UTRA” and “not served by NR”:

-   -   Includes the radio parameters per PC5 RAT (i.e. LTE PC5, NR PC5)         with Geographical Area(s) and an indication of whether they are         “operator managed” or “non-operator managed”. These radio         parameters (e.g., frequency bands) are defined in TS 36.331 [14]         and TS 38.331 [15]. The UE uses the radio parameters to perform         V2X communications over PC5 reference point when “not served by         E-UTRA” and “not served by NR” only if the UE can reliably         locate itself in the corresponding Geographical Area. Otherwise,         the UE is not authorized to transmit.     -   NOTE 2: Whether a frequency band is “operator managed” or         “non-operator managed” in a given Geographical Area is defined         by local regulations.

3) Policy/parameters per RAT for PC5 Tx Profile selection:

-   -   The mapping of V2X service types (e.g. PSIDs or ITS-AIDS) to Tx         Profiles (see TS 36.300 [9] and TS 38.300 [11] for further         information).

4) Policy/parameters related to privacy:

-   -   The list of V2X service types, e.g. PSIDs or ITS-AIDS of the V2X         applications, with Geographical Area(s) that require privacy         support.     -   A privacy timer value indicating the duration after which the UE         shall change each source Layer-2 ID self-assigned by the UE when         privacy is required.

5) Policy/parameters when LTE PC5 is selected:

-   -   Same as specified in TS 23.285 [8] clause 4.4.1.1.2 item 3)         Policy/parameters except for the mapping of V2X service types to         Tx Profiles and the list of V2X services with Geographical         Area(s) that require privacy support.

6) Policy/parameters when NR PC5 is selected:

-   -   The mapping of V2X service types (e.g. PSIDs or ITS-AIDS) to V2X         frequencies with Geographical Area(s).     -   The mapping of Destination Layer-2 ID(s) and the V2X service         types, e.g. PSIDs or ITS-AIDs of the V2X application for         broadcast.     -   The mapping of Destination Layer-2 ID(s) and the V2X service         types, e.g. PSIDs or ITS-AIDs of the V2X application for         groupcast.     -   The mapping of default Destination Layer-2 ID(s) for initial         signalling to establish unicast connection and the V2X service         types, e.g. PSIDs or ITS-AIDS of the V2X application.     -   NOTE 3: The same default Destination Layer-2 ID for unicast         initial signalling can be mapped to more than one V2X service         types. In the case where different V2X services are mapped to         distinct default Destination Layer-2 IDs, when the UE intends to         establish a single unicast link that can be used for more than         one V2X service types, the UE can select any of the default         Destination Layer-2 IDs to use for the initial signalling.     -   PC5 QoS mapping configuration:         -   Input from V2X application layer:             -   V2X service type (e.g. PSID or ITS-AID).             -   (Optional) V2X Application Requirements for the V2X                 service type, e.g. priority requirement, reliability                 requirement, delay requirement, range requirement.     -   NOTE 4: Details of V2X Application Requirements for the V2X         service type is up to implementation and out of scope of this         specification.     -   Output:         -   PC5 QoS parameters defined in clause 5.4.2 (i.e. PQI and             conditionally other parameters such as MFBR/GFBR, etc.).     -   AS layer configurations (see TS 38.331 [15]), e.g. the mapping         of PC5 QoS profile(s) to radio bearer(s), when the UE is “not         served by E-UTRA” and “not served by NR”.         -   The PC5 QoS profile contains PC5 QoS parameters described in             clause 5.4.2, and value for the QoS characteristics             regarding Priority Level, Averaging Window, Maximum Data             Burst Volume if default value is not used as defined in             Table 5.4.4-1.

7) Validity timer indicating the expiration time of the V2X Policy/Parameter.

The above parameter sets from bullet 2) to 6) may be configured in the UE through the V1 reference point by the V2X Application Server. [ . . . ]

5.2.1.4 Unicast Mode Communication Over PC5 Reference Point

Unicast mode of communication is only supported over NR based PC5 reference point. FIG. 5.2.1.4-1 illustrates an example of PC5 unicast links.

FIG. 5.2.1.4-1 of 3GPP TS 23.287 V16.2.0, Entitled “Example of PC5 Unicast Links”, is Reproduced as FIG. 5

The following principles apply when the V2X communication is carried over PC5 unicast link:

-   -   A PC5 unicast link between two UEs allows V2X communication         between one or more pairs of peer V2X services in these UEs. All         V2X services in the UE using the same PC5 unicast link use the         same Application Layer ID.     -   NOTE 1: An Application Layer ID can change in time as described         in clauses 5.6.1.1 and 6.3.3.2, due to privacy. This does not         cause a re-establishment of a PC5 unicast link. The UE triggers         a Link Identifier Update procedure as specified in clause         6.3.3.2.     -   One PC5 unicast link supports one or more V2X service types         (e.g. PSIDs or ITS-AIDS) if these V2X service types are at least         associated with the pair of peer Application Layer IDs for this         PC5 unicast link. For example, as illustrated in FIG. 5.2.1.4-1,         UE A and UE B have two PC5 unicast links, one between peer         Application Layer ID 1/UE A and Application Layer ID 2/UE B and         one between peer Application Layer ID 3/UE A and Application         Layer ID 4/UE B.     -   NOTE 2: A source UE is not required to know whether different         target Application Layer IDs over different PC5 unicast links         belong to the same target UE.     -   A PC5 unicast link supports V2X communication using a single         network layer protocol e.g. IP or non-IP.     -   A PC5 unicast link supports per-flow QoS model as specified in         clause 5.4.1.         When the Application layer in the UE initiates data transfer for         a V2X service type which requires unicast mode of communication         over PC5 reference point:     -   the UE shall reuse an existing PC5 unicast link if the pair of         peer Application Layer IDs and the network layer protocol of         this PC5 unicast link are identical to those required by the         application layer in the UE for this V2X service, and modify the         existing PC5 unicast link to add this V2X service type as         specified in clause 6.3.3.4; otherwise     -   the UE shall trigger the establishment of a new PC5 unicast link         as specified in clause 6.3.3.1.         After successful PC5 unicast link establishment, UE A and UE B         use the same pair of Layer-2 IDs for subsequent PC5-S signalling         message exchange and V2X service data transmission as specified         in clause 5.6.1.4. The V2X layer of the transmitting UE         indicates to the AS layer whether a transmission is for a PC5-S         signalling message (i.e. Direct Communication Request/Accept,         Link Identifier Update Request/Response/Ack, Disconnect         Request/Response, Link Modification Request/Accept) or V2X         service data.         For every PC5 unicast link, a UE self-assigns a distinct PC5         Link Identifier that uniquely identifies the PC5 unicast link in         the UE for the lifetime of the PC5 unicast link. Each PC5         unicast link is associated with a Unicast Link Profile which         includes:     -   V2X service type(s) (e.g. PSID(s) or ITS-AID(s)); and     -   Application Layer ID and Layer-2 ID of UE A; and     -   Application Layer ID and Layer-2 ID of UE B; and     -   network layer protocol used on the PC5 unicast link; and     -   for each V2X service type, a set of PC5 QoS Flow Identifier(s)         (PFI(s)). Each PFI is associated with QoS parameters (i.e. PQI).         For privacy reason, the Application Layer IDs and Layer-2 IDs         may change as described in clauses 5.6.1.1 and 6.3.3.2 during         the lifetime of the PC5 unicast link and, if so, shall be         updated in the Unicast Link Profile accordingly. The UE uses PC5         Link Identifier to indicate the PC5 unicast link to V2X         Application layer, therefore V2X Application layer identifies         the corresponding PC5 unicast link even if there are more than         one unicast link associated with one V2X service type (e.g. the         UE establishes multiple unicast links with multiple UEs for a         same V2X service type).         The Unicast Link Profile shall be updated accordingly after a         Layer-2 link modification for an established PC5 unicast link as         specified in clause 6.3.3.4 or Layer-2 link identifier update as         specified in clause 6.3.3.2.         V2X Service Info and QoS Info are carried in PC5-S signalling         messages and exchanged between two UEs as specified in clause         6.3.3. Based on the exchanged information, PFI is used to         identify V2X service. When the receiving UE receives V2X service         data over the established PC5 unicast link, the receiving UE         determines the appropriate V2X service based on the PFI to         forward the received V2X service data to the upper layer.         Upon receiving an indication from the AS layer that the PC5-RRC         connection was released due to RLF, the V2X layer in the UE         locally releases the PC5 unicast link associated with this         PC5-RRC connection. The AS layer uses PC5 Link Identifier to         indicate the PC5 unicast link whose PC5-RRC connection was         released.         When the PC5 unicast link has been released as specified in         clause 6.3.3.3, the V2X layer of each UE for the PC5 unicast         link informs the AS layer that the PC5 unicast link has been         released. The V2X layer uses PC5 Link Identifier to indicate the         released unicast link.         [ . . . ]

5.6.1.4 Identifiers for Unicast Mode V2X Communication Over PC5 Reference Point

For unicast mode of V2X communication over PC5 reference point, the destination Layer-2 ID used depends on the communication peer. The Layer-2 ID of the communication peer, identified by the Application Layer ID, may be discovered during the establishment of the PC5 unicast link, or known to the UE via prior V2X communications, e.g. existing or prior unicast link to the same Application Layer ID, or obtained from application layer service announcements. The initial signalling for the establishment of the PC5 unicast link may use the known Layer-2 ID of the communication peer, or a default destination Layer-2 ID associated with the V2X service type (e.g. PSID/ITS-AID) configured for PC5 unicast link establishment, as specified in clause 5.1.2.1. During the PC5 unicast link establishment procedure, Layer-2 IDs are exchanged, and should be used for future communication between the two UEs, as specified in clause 6.3.3.1. The Application Layer ID is associated with one or more V2X applications within the UE. If UE has more than one Application Layer IDs, each Application Layer ID of the same UE may be seen as different UE's Application Layer ID from the peer UE's perspective. The UE maintains a mapping between the Application Layer IDs and the source Layer-2 IDs used for the PC5 unicast links, as the V2X application layer does not use the Layer-2 IDs. This allows the change of source Layer-2 ID without interrupting the V2X applications. When Application Layer IDs change, the source Layer-2 ID(s) of the PC5 unicast link(s) shall be changed if the link(s) was used for V2X communication with the changed Application Layer IDs. Based on privacy configuration as specified in clause 5.1.2.1, the update of the new identifiers of a source UE to the peer UE for the established unicast link may cause the peer UE to change its Layer-2 ID and optionally IP address/prefix if IP communication is used as defined in clause 6.3.3.2. A UE may establish multiple PC5 unicast links with a peer UE and use the same or different source Layer-2 IDs for these PC5 unicast links. [ . . . ]

6.1 Control and User Plane Stacks 6.1.1 User Plane for NR PC5 Reference Point Supporting V2X Services

Fig. 6.1.1-1 depicts a user plane for NR PC5 reference point, i.e. PC5 User Plane Protocol stack.

FIG. 6.1.1-1 of 3GPP TS 23.287 V16.2.0, Entitled “User Plane for NR PC5 Reference Point”, is Reproduced as FIG. 6

IP and Non-IP PDCP SDU types are supported for the V2X communication over PC5 reference point. For IP PDCP SDU type, only IPv6 is supported. The IP address allocation and configuration are as defined in clause 5.6.1.1. The Non-IP PDCP SDU contains a Non-IP Type header, which indicates the V2X message family used by the application layer, e.g. IEEE 1609 family's WSMP [18], ISO defined FNTP [19].

-   -   NOTE: The Non-IP Type header and allowed values are defined in         TS 24.587 [24].         The packets from V2X application layer are handled by the V2X         layer before transmitting them to the AS layer, e.g. V2X layer         maps the IP/Non IP packets to PC5 QoS Flow and marks the         corresponding PFI.

6.1.2 Control Plane for NR PC5 Reference Point Supporting V2X Services

-   Editor's note: Whether PC5-S messages are carried in PC5 RRC     signalling depends on RAN decision.     FIG. 6.1.2-1 depicts a control plane for NR PC5 reference point,     i.e. PC5 Signalling Protocol stack.

FIG. 6.1.2-1 of 3GPP TS 23.287 V16.2.0, Entitled “Control Plane for NR PC5 Reference Point”, is Reproduced as FIG. 7

[ . . . ]

6.3.3 Unicast Mode V2X Communication Over PC5 Reference Point 6.3.3.1 Layer-2 Link Establishment Over PC5 Reference Point

To perform unicast mode of V2X communication over PC5 reference point, the UE is configured with the related information as described in clause 5.1.2.1. FIG. 6.3.3.1-1 shows the layer-2 link establishment procedure for unicast mode of V2X communication over PC5 reference point.

FIG. 6.3.3.1-1 of 3GPP TS 23.287 V16.2.0, Entitled “Layer-2 Link Establishment Procedure”, is Reproduced as FIG. 8

-   -   1. The UE(s) determine the destination Layer-2 ID for signalling         reception for PC5 unicast link establishment as specified in         clause 5.6.1.4. The destination Layer-2 ID is configured with         the UE(s) as specified in clause 5.1.2.1.     -   2. The V2X application layer in UE-1 provides application         information for PC5 unicast communication. The application         information includes the V2X service type(s) (e.g. PSID(s) or         ITS-AID(s)) of the V2X application and the initiating UE's         Application Layer ID. The target UE's Application Layer ID may         be included in the application information.         -   The V2X application layer in UE-1 may provide V2X             Application Requirements for this unicast communication.             UE-1 determines the PC5 QoS parameters and PFI as specified             in clause 5.4.1.4.         -   If UE-1 decides to reuse the existing PC5 unicast link as             specified in clause 5.2.1.4, the UE triggers Layer-2 link             modification procedure as specified in clause 6.3.3.4.     -   3. UE-1 sends a Direct Communication Request message to initiate         the unicast layer-2 link establishment procedure. The Direct         Communication Request message includes:         -   Source User Info: the initiating UE's Application Layer ID             (i.e. UE-1's Application Layer ID).         -   If the V2X application layer provided the target UE's             Application Layer ID in step 2, the following information is             included:             -   Target User Info: the target UE's Application Layer ID                 (i.e. UE-2's Application Layer ID).         -   V2X Service Info: the information about V2X Service(s)             requesting Layer-2 link establishment (e.g. PSID(s) or             ITS-AID(s)).         -   Security Information: the information for the establishment             of security.     -   NOTE 1: The Security Information and the necessary protection of         the Source User Info and Target User Info are defined by SA WG3.         -   The source Layer-2 ID and destination Layer-2 ID used to             send the Direct Communication Request message are determined             as specified in clauses 5.6.1.1 and 5.6.1.4. The destination             Layer-2 ID may be broadcast or unicast Layer-2 ID. When             unicast Layer-2 ID is used, the Target User Info shall be             included in the Direct Communication Request message.         -   UE-1 sends the Direct Communication Request message via PC5             broadcast or unicast using the source Layer-2 ID and the             destination Layer-2 ID.     -   4. Security with UE-1 is established as below:         -   4a. If the Target User Info is included in the Direct             Communication Request message, the target UE, i.e. UE-2,             responds by establishing the security with UE-1.         -   4b. If the Target User Info is not included in the Direct             Communication Request message, the UEs that are interested             in using the announced V2X Service(s) over a PC5 unicast             link with UE-1 responds by establishing the security with             UE-1.     -   NOTE 2: The signalling for the Security Procedure is defined by         SA WG3.         -   When the security protection is enabled, UE-1 sends the             following information to the target UE:             -   If IP communication is used:                 -   IP Address Configuration: For IP communication, IP                     address configuration is required for this link and                     indicates one of the following values:                 -    “IPv6 Router” if IPv6 address allocation mechanism                     is supported by the initiating UE, i.e., acting as                     an IPv6 Router; or                 -    “IPv6 address allocation not supported” if IPv6                     address allocation mechanism is not supported by the                     initiating UE.                 -   Link Local IPv6 Address: a link-local IPv6 address                     formed locally based on RFC 4862 [21] if UE-1 does                     not support the IPv6 IP address allocation                     mechanism, i.e. the IP Address Configuration                     indicates “IPv6 address allocation not supported”.             -   QoS Info: the information about PC5 QoS Flow(s). For                 each PC5 QoS Flow, the PFI and the corresponding PC5 QoS                 parameters (i.e. PQI and conditionally other parameters                 such as MFBR/GFBR, etc.).         -   The source Layer-2 ID used for the security establishment             procedure is determined as specified in clauses 5.6.1.1 and             5.6.1.4. The destination Layer-2 ID is set to the source             Layer-2 ID of the received Direct Communication Request             message.         -   Upon receiving the security establishment procedure             messages, UE-1 obtains the peer UE's Layer-2 ID for future             communication, for signalling and data traffic for this             unicast link.     -   5. A Direct Communication Accept message is sent to UE-1 by the         target UE(s) that has successfully established security with         UE-1:         -   5a. (UE oriented Layer-2 link establishment) If the Target             User Info is included in the Direct Communication Request             message, the target UE, i.e. UE-2 responds with a Direct             Communication Accept message if the Application Layer ID for             UE-2 matches.         -   5b. (V2X Service oriented Layer-2 link establishment) If the             Target User Info is not included in the Direct Communication             Request message, the UEs that are interested in using the             announced V2X Service(s) respond to the request by sending a             Direct Communication Accept message (UE-2 and UE-4 in FIG.             6.3.3.1-1).         -   The Direct Communication Accept message includes:             -   Source User Info: Application Layer ID of the UE sending                 the Direct Communication Accept message.             -   QoS Info: the information about PC5 QoS Flow(s). For                 each PC5 QoS Flow, the PFI and the corresponding PC5 QoS                 parameters requested by UE-1 (i.e. PQI and conditionally                 other parameters such as MFBR/GFBR, etc).             -   If IP communication is used:                 -   IP Address Configuration: For IP communication, IP                     address configuration is required for this link and                     indicates one of the following values:                 -    “IPv6 Router” if IPv6 address allocation mechanism                     is supported by the target UE, i.e., acting as an                     IPv6 Router; or                 -    “IPv6 address allocation not supported” if IPv6                     address allocation mechanism is not supported by the                     target UE.                 -   Link Local IPv6 Address: a link-local IPv6 address                     formed locally based on RFC 4862 [21] if the target                     UE does not support the IPv6 IP address allocation                     mechanism, i.e. the IP Address Configuration                     indicates “IPv6 address allocation not supported”,                     and UE-1 included a link-local IPv6 address in the                     Direct Communication Request message. The target UE                     shall include a non-conflicting link-local IPv6                     address.     -   If both UEs (i.e. the initiating UE and the target UE) selected         to use link-local IPv6 address, they shall disable the duplicate         address detection defined in RFC 4862 [21].     -   NOTE 3: When either the initiating UE or the target UE indicates         the support of IPv6 router, corresponding address configuration         procedure would be carried out after the establishment of the         layer 2 link, and the link-local IPv6 addresses are ignored. The         V2X layer of the UE that established PC5 unicast link passes the         PC5 Link Identifier assigned for the unicast link and the PC5         unicast link related information down to the AS layer. The PC5         unicast link related information includes Layer-2 ID information         (i.e. source Layer-2 ID and destination Layer-2 ID). This         enables the AS layer to maintain the PC5 Link Identifier         together with the PC5 unicast link related information.     -   6. V2X service data is transmitted over the established unicast         link as below:         -   The PC5 Link Identifier, and PFI are provided to the AS             layer, together with the V2X service data.         -   Optionally in addition, the Layer-2 ID information (i.e.             source Layer-2 ID and destination Layer-2 ID) is provided to             the AS layer.     -   NOTE 4: It is up to UE implementation to provide the Layer-2 ID         information to the AS layer.         -   UE-1 sends the V2X service data using the source Layer-2 ID             (i.e. UE-1's Layer-2 ID for this unicast link) and the             destination Layer-2 ID (i.e. the peer UE's Layer-2 ID for             this unicast link).     -   NOTE 5: PC5 unicast link is bi-directional, therefore the peer         UE of UE-1 can send the V2X service data to UE-1 over the         unicast link with UE-1.

6.3.3.3 Layer-2 Link Release Over PC5 Reference Point

FIG. 6.3.3.3-1 shows the layer-2 link release procedure over PC5 reference point.

FIG. 6.3.3.3-1 of 3GPP TS 23.287 V16.2.0, Entitled “Layer-2 Link Release Procedure”, is Reproduced as FIG. 9

-   -   0. UE-1 and UE-2 have a unicast link established as described in         clause 6.3.3.1.     -   1. UE-1 sends a Disconnect Request message to UE-2 in order to         release the layer-2 link and deletes all context data associated         with the layer-2 link.     -   2. Upon reception of the Disconnect Request message UE-2 may         respond with a Disconnect Response message and deletes all         context data associated with the layer-2 link.         -   The V2X layer of each UE informs the AS layer that the             unicast link has been released. The V2X layer uses PC5 Link             Identifier to indicate the released unicast link. This             enables the AS layer to delete the context related to the             released unicast link.

6.3.3.4 Layer-2 Link Modification for a Unicast Link

FIG. 6.3.3.4-1 shows the layer-2 link modification procedure for a unicast link. This procedure is used to:

-   -   add new V2X service(s) to the existing PC5 unicast link.     -   remove V2X service(s) from the existing PC5 unicast link.     -   add new PC5 QoS Flow(s) in the existing PC5 unicast link.     -   modify existing PC5 QoS Flow(s) in the existing PC5 unicast         link.     -   remove existing PC5 QoS Flow(s) in the existing PC5 unicast         link.

FIG. 6.3.3.4-1 of 3GPP TS 23.287 V16.2.0, Entitled “Layer-2 Link Modification Procedure”, is Reproduced as FIG. 10

-   -   0. UE-1 and UE-2 have a unicast link established as described in         clause 6.3.3.1.     -   1. The V2X application layer in UE-1 provides application         information for PC5 unicast communication. The application         information includes the V2X service type(s) (e.g. PSID(s) or         ITS-AID(s)) of the V2X application(s) and the initiating UE's         Application Layer ID. The target UE's Application Layer ID may         be included in the application information. If UE-1 decides to         reuse the existing PC5 unicast link as specified in clause         5.2.1.4, so decides to modify the unicast link established with         UE-2, UE-1 sends a Link Modification Request to UE-2.         -   The Link Modification Request message includes:             -   a) To add new V2X service(s) to the existing PC5 unicast                 link:                 -   V2X Service Info: the information about V2X                     Service(s) to be added (e.g. PSID(s) or ITS-AID(s)).                 -   QoS Info: the information about PC5 QoS Flow(s) for                     each V2X Service to be added. For each PC5 QoS Flow,                     the PFI and the corresponding PC5 QoS parameters                     (i.e. PQI and conditionally other parameters such as                     MFBR/GFBR, etc).             -   b) To remove a V2X service(s) from the the existing PC5                 unicast link:                 -   V2X Service Info: the information about V2X                     Service(s) to be removed (e.g. PSID(s) or                     ITS-AID(s)).             -   c) To add new PC5 QoS Flow(s) in the existing PC5                 unicast link:                 -   V2X Service Info: the information about V2X                     Service(s) that needs to add new QoS Flows (e.g.                     PSID(s) or ITS-AID(s)).                 -   QoS Info: the information about PC5 QoS Flow(s) to                     be modified. For each PC5 QoS Flow, the PFI and the                     corresponding PC5 QoS parameters (i.e. PQI and                     conditionally other parameters such as MFBR/GFBR,                     etc).             -   d) To modify PC5 QoS Flow(s) in the existing PC5 unicast                 link:                 -   QoS Info: the information about PC5 QoS Flow(s) to                     be modified. For each PC5 QoS Flow, the PFI and the                     corresponding PC5 QoS parameters (i.e. PQI and                     conditionally other parameters such as MFBR/GFBR,                     etc.).             -   e) To remove PC5 QoS Flow(s) in the existing PC5 unicast                 link:                 -   PFIs.     -   2. UE-2 responds with a Link Modification Accept message.         -   The Link Modification Accept message includes:             -   For case a), case c) and case d) described in step 1:                 -   QoS Info: the information about PC5 QoS Flow(s). For                     each PC5 QoS Flow, the PFI and the corresponding PC5                     QoS parameters (i.e. PQI and conditionally other                     parameters such as MFBR/GFBR, etc).         -   The V2X layer of each UE provides information about the             unicast link modification to the AS layer. This enables the             AS layer to update the context related to the modified             unicast link.

6.3.3.5 Layer-2 Link Maintenance Over PC5 Reference Point

The PC5 Signalling Protocol shall support keep-alive functionality that is used to detect if a particular PC5 unicast link is still valid. Either side of the PC5 unicast link can initiate the layer-2 link maintenance procedure (i.e. keep-alive procedure), based on for example triggers from the AS layer or internal timers. The UEs shall minimize the keep-alive signalling, e.g. cancel the procedure if data are successfully received over the PC5 unicast link.

FIG. 6.3.3.5-1 of 3GPP TS 23.287 V16.2.0, Entitled “Layer-2 Link Maintenance Procedure”, is Reproduced as FIG. 11

-   -   0. UE-1 and UE-2 have a unicast link established as described in         clause 6.3.3.1.     -   1. Based on trigger conditions, UE-1 sends a Keep-alive message         to UE-2 in order to determine the status of the PC5 unicast         link.     -   NOTE 1: It is left to Stage 3 to determine the exact triggers         for the keep-alive messages. For example, the trigger can be         based on a timer associated with the Layer-2 link. The timer can         be reset with a successful reception event defined by TS 38.300         [11].     -   2. Upon reception of the Keep-alive message, UE-2 responds with         a Keep-alive Ack message.

The UE initiating the keep-alive procedure shall determine the follow up actions based on the result of the signalling, e.g. proceed with implicit layer-2 link release.

-   -   NOTE 2: It is Left to Stage 3 to Determine the Follow Up         Actions. For Example, a Successful reception event can also         cancel the layer-2 link release if received in time.

3GPP TS 24.587 specifies Stage 3 PC5 unicast link establishment procedure as follows:

6.1.2.2 PC5 Unicast Link Establishment Procedure 6.1.2.2.1 General

The PC5 unicast link establishment procedure is used to establish a PC5 unicast link between two UEs. The UE sending the request message is called the “initiating UE” and the other UE is called the “target UE”.

-   -   Editor's note: The details about security procedure defined by         SA3 are FFS.     -   Editor's note: The details of the IEs of the following messages         are FFS.

6.1.2.2.2 PC5 Unicast Link Establishment Procedure Initiation by Initiating UE

-   -   Editor's note: This section needs to be revisited after SA3 have         determined the full set of security requirements for unicast         link establishment.         The initiating UE shall meet the following pre-conditions before         initiating this procedure:     -   a) a request from upper layers to transmit the packet for V2X         service over PC5;     -   b) the link layer identifier for the initiating UE (i.e. layer 2         ID used for unicast communication) is available (e.g.         pre-configured or self-assigned);     -   c) the link layer identifier for the unicast initial signaling         (i.e. destination layer 2 ID used for unicast initial signaling)         is available to the initiating UE (e.g. pre-configured, obtained         as specified in clause 5.2.3 or known via prior V2X         communication);     -   d) the initiating UE is either authorised for V2X communication         over PC5 in NR in the serving PLMN, or has a valid authorization         for V2X communication over PC5 in NR when not served by E-UTRAN         and not served by NR; and     -   e) there is no existing PC5 unicast link for the pair of peer         application layer IDs and the network layer protocol of this PC5         unicast link are identical to those required by the upper layer         in the initiating UE for this V2X service.         In order to initiate the PC5 unicast link establishment         procedure, the initiating UE shall create a DIRECT LINK         ESTABLISHMENT REQUEST message. The initiating UE:     -   a) shall include the source user info set to the initiating UE's         application layer ID received from upper layers;     -   b) shall include the V2X service identifier received from upper         layer;     -   c) may include the target user info set to the target UE's         application layer ID if received from upper layers; and     -   d) shall include the security establishment information.     -   Editor's note: The parameters in the security establishment         information will be defined by SA3.         After the DIRECT LINK ESTABLISHMENT REQUEST message is         generated, the initiating UE shall pass this message to the         lower layers for transmission along with the initiating UE's         Layer 2 ID for unicast communication and the destination layer 2         ID used for unicast initial signaling, and start timer T5000.         The UE shall not send a new DIRECT LINK ESTABLISHMENT REQUEST         message to the same target UE identified by the same application         layer ID while timer T5000 is running.

FIG. 6.1.2.2.2 of 3GPP TS 24.587 V16.0.0, Entitled “PC5 Unicast Link Establishment Procedure”, is Reproduced as FIG. 12 6.1.2.2.3 PC5 Unicast Link Establishment Procedure Accepted by the Target UE

Upon receipt of a DIRECT LINK ESTABLISHMENT REQUEST message, the target UE shall assign a layer-2 ID for this PC5 unicast link and store this assigned layer-2 ID and the source layer 2 ID used in the transport of this message provided by the lower layers. This pair of layer-2 IDs is associated with a PC5 unicast link context.

If:

-   -   a) the target user info IE is included in the DIRECT LINK         ESTABLISHMENT REQUEST message and this IE includes the target         UE's application layer ID; or     -   b) the target user info IE is not included in the DIRECT LINK         ESTABLISHMENT REQUEST message and the target UE is interested in         the V2X service identified by the V2X service identifier in the         DIRECT LINK ESTABLISHMENT REQUEST message;         then the target UE shall either identify an existing security         context with the initiating UE, or establish a new security         context by performing one or more PC5 unicast link         authentication procedures as specified in clause 6.1.2.6, and         performing the PC5 unicast link security mode control procedure         as specified in clause 6.1.2.7.         Upon successful completion of the PC5 unicast link security mode         control procedure, in order to determine whether the DIRECT LINK         ESTABLISHMENT REQUEST message can be accepted or not, in case of         IP communication, the target UE checks whether there is at least         one common IP address configuration option supported by both the         initiating UE and the target UE.         If the target UE accepts the PC5 unicast link establishment         procedure, the target UE shall create a DIRECT LINK         ESTABLISHMENT ACCEPT message. The target UE:     -   a) shall include the source user info set to the target UE's         application layer ID received from upper layers;     -   b) shall include a PQFI and the corresponding PC5 QoS         parameters;     -   c) may include an IP address configuration IE set to one of the         following values if IP communication is used:         -   1) “IPv6 router” if only IPv6 address allocation mechanism             is supported by the target UE, i.e. acting as an IPv6             router; or         -   2) “IPv6 address allocation not supported” if IPv6 address             allocation mechanism is not supported by the target UE;     -   d) may include a link local IPv6 address IE formed locally based         on IETF RFC 4862 [16] if IP address configuration IE is set to         “IPv6 address allocation not supported” and the received DIRECT         LINK ESTABLISHMENT REQUEST message included a link local IPv6         address IE.

6.1.2.2.4 PC5 Unicast Link Establishment Procedure Completion by the Initiating UE

Upon receipt of the DIRECT LINK ESTABLISHMENT ACCEPT message, the initiating UE shall stop timer T5000 and store the source layer-2 ID and the destination Layer-2 ID used in the transport of this message provided by the lower layers. This pair of layer-2 IDs shall be associated with a PC5 unicast link context. From this time onward the initiating UE shall use the established link for V2X communication over PC5 and additional PC5 signalling messages to the target UE.

6.1.2.2.5 PC5 Unicast Link Establishment Procedure not Accepted by the Target UE

If the DIRECT LINK ESTABLISHMENT REQUEST message cannot be accepted, the target UE shall send a DIRECT LINK ESTABLISHMENT REJECT message. The DIRECT LINK ESTABLISHMENT REJECT message contains a PC5 signalling protocol cause IE set to one of the following cause values:

-   -   #1 direct communication to the target UE not allowed;     -   #3 conflict of Layer 2 ID for unicast communication is detected;     -   #5 lack of resources for proposed link; or     -   #111 protocol error, unspecified.         If the target UE is not allowed to accept this request e.g.         based on operator policy or service authorisation provisioning,         the target UE shall send a DIRECT LINK ESTABLISHMENT REJECT         message containing PC5 signalling protocol cause value #1         “direct communication to the target UE not allowed”.         For a received DIRECT LINK ESTABLISHMENT REQUEST message from a         Layer 2 ID (for unicast communication), if the target UE already         has an existing link established to the UE known to use this         Layer 2 ID or is currently processing a DIRECT LINK         ESTABLISHMENT REQUEST message from the same Layer 2 ID, but with         user info different from the user info IE included in this new         incoming message, the target UE shall send a DIRECT LINK         ESTABLISHMENT REJECT message containing PC5 signalling protocol         cause value #3 “conflict of Layer 2 ID for unicast communication         is detected”.         If the PC5 unicast link establishment fails due to the         congestion problems or other temporary lower layer problems         causing resource constraints, the target UE shall send a DIRECT         LINK ESTABLISHMENT REJECT message containing PC5 signalling         protocol cause value #5 “lack of resources for proposed link”.         For other reasons that causing the failure of link         establishment, the target UE shall send a DIRECT LINK         ESTABLISHMENT REJECT message containing PC5 signalling protocol         cause value #111 “protocol error, unspecified”.         Upon receipt of the DIRECT LINK ESTABLISHMENT REJECT message,         the initiating UE shall stop timer T5000 and abort the PC5         unicast link establishment procedure. If the PC5 signalling         protocol cause value in the DIRECT LINK ESTABLISHMENT REJECT         message is #1 “direct communication to the target UE not         allowed” or #5 “lack of resources for proposed link”, then the         UE shall not attempt to start PC5 unicast link establishment         with the same target UE at least for a time period T.     -   NOTE: The length of time period T is UE implementation specific         and can be different for the case when the UE receives PC5         signalling protocol cause value #1 “direct communication to the         target UE not allowed” or when the UE receives PC5 signalling         protocol cause value #5 “lack of resources for proposed link”.

6.1.2.2.6 Abnormal Cases 6.1.2.2.6.1 Abnormal Cases at the Initiating UE

If timer T5000 expires, the initiating UE shall retransmit the DIRECT LINK ESTABLISHMENT REQUEST message and restart timer T5000. After reaching the maximum number of allowed retransmissions, the initiating UE shall abort the PC5 unicast link establishment procedure and may notify the upper layer that the target UE is unreachable.

-   -   NOTE: The maximum number of allowed retransmissions is UE         implementation specific.         If the need to establish a link no longer exists before the         procedure is completed, the initiating UE shall abort the         procedure.

6.1.2.2.6.2 Abnormal Cases at the Target UE

For a received DIRECT LINK ESTABLISHMENT REQUEST message from a source Layer 2 ID (for unicast communication), if the target UE already has an existing link established to the UE known to use this source Layer 2 ID and the new request contains an identical source user info as the known user, the UE shall process the new request. However, the target UE shall only delete the existing link context after the new link establishment procedure succeeds.

3GPP TS 38.331 specifies Radio Resource Control (RRC) reconfiguration, UE capability information, sidelink UE information, and sidelink Data Radio Bearer (DRB) establishment as follows:

5.8 Sidelink 5.8.1 General

NR sidelink communication consists of unicast, groupcast and broadcast. The PC5-RRC connection is a logical connection between a pair of a Source Layer-2 ID and a Destination Layer-2 ID in the AS. The PC5-RRC signalling, as specified in sub-clause 5.8.9, can be initiated after its corresponding PC5 unicast link establishment (TS 23.287 [55]). The PC5-RRC connection and the corresponding sidelink SRBs and sidelink DRBs are released when the PC5 unicast link is released as indicated by upper layers. For each PC5-RRC connection of unicast, one sidelink SRB is used to transmit the PC5-S messages before the PC5-S security has been established. One sidelink SRB is used to transmit the PC5-S messages to establish the PC5-S security. One sidelink SRB is used to transmit the PC5-S messages after the PC5-S security has been established, which is protected. One sidelink SRB is used to transmit the PC5-RRC signalling, which is protected and only sent after the PC5-S security has been established.

-   -   NOTE 1: In case the configurations for NR sidelink communication         are acquired via the E-UTRA, the configurations for NR sidelink         communication in SIB12 and sl-ConfigDedicatedNR within         RRCReconfiguration used in subclause 5.8 are provided by the         configurations in SystemInformationBlockTypeXX2 and         sl-ConfigDedicatedNR within RRCConnectionReconfiguration as         specified in TS 36.331 [10], respectively.         [ . . . ]

5.8.9.1 Sidelink RRC Reconfiguration 5.8.9.1.1 General FIG. 5.8.9.1.1-1 of 3GPP TS 38.331 V16.0.0, Entitled “Sidelink RRC Reconfiguration, Successful”, is Reproduced as FIG. 13 FIG. 5.8.9.1.1-2 of 3GPP TS 38.331 V16.0.0, Entitled “Sidelink RRC Reconfiguration, Failure”, is Reproduced as FIG. 14

The purpose of this procedure is to establish/modify/release sidelink DRBs or configure NR sidelink measurement and report for a PC5-RRC connection. The UE may initiate the sidelink RRC reconfiguration procedure and perform the operation in sub-clause 5.8.9.1.2 to its peer UE in following cases:

-   -   the release of sidelink DRBs associated with the peer UE, as         specified in sub-clause 5.8.9.1.4;     -   the establishment of sidelink DRBs associated with the peer UE,         as specified in sub-clause 5.8.9.1.5;     -   the modification for the parameters included in SLRB-Config of         sidelink DRBs associated with the peer UE, as specified in         sub-clause 5.8.9.1.5;     -   the configuration of the peer UE to perform NR sidelink         measurement and report.

5.8.9.1.2 Actions Related to Transmission of RRCReconfigurationSidelink Message

The UE shall set the contents of RRCReconfigurationSidelink message as follows:

-   -   1> for each sidelink DRB that is to be released, according to         sub-clause 5.8.9.1.4.1, due to configuration by         sl-ConfigDedicatedNR, SIB12, SidelinkPreconfigNR or by upper         layers:     -   2> set the slrb-PC5-ConfigIndex included in the         slrb-ConfigToReleaseList corresponding to the sidelink DRB;     -   1> for each sidelink DRB that is to be established or modified,         according to sub-clause 5.8.9.1.5.1, due to receiving         sl-ConfigDedicatedNR, SIB12, SidelinkPreconfigNR:     -   2> set the SLRB-Config included in the slrb-ConfigToAddModList,         according to the received sl-RadioBearerConfig and         sl-RLC-BearerConfig corresponding to the sidelink DRB;     -   1> for each NR sidelink measurement and report that is to be         configured:     -   2> set the sl-MeasConfig according to the stored NR sidelink         measurement configuration information;     -   1> start timer T400 for the destination associated with the         sidelink DRB;         The UE shall submit the RRCReconfigurationSidelink message to         lower layers for transmission.

5.8.9.1.3 Reception of an RRCReconfigurationSidelink by the UE

The UE shall perform the following actions upon reception of the RRCReconfigurationSidelink:

-   -   1> if the RRCReconfigurationSidelink includes the         slrb-ConfigToReleaseList:     -   2> for each slrb-PC5-ConfigIndex value included in the         slrb-ConfigToReleaseList that is part of the current UE sidelink         configuration;     -   3> perform the sidelink DRB release procedure, according to         sub-clause 5.8.9.1.4;     -   1> if the RRCReconfigurationSidelink includes the         slrb-ConfigToAddModList:     -   2> for each slrb-PC5-ConfigIndex value included in the         slrb-ConfigToAddModList that is not part of the current UE         sidelink configuration:     -   3> apply the sl-MappedQoS-FlowsToAddList, if included;     -   3> perform the sidelink DRB addition procedure, according to         sub-clause 5.8.9.1.5;     -   2> for each slrb-PC5-ConfigIndex value included in the         slrb-ConfigToAddModList that is part of the current UE sidelink         configuration:     -   3> apply the sl-MappedQoS-FlowsToAddList and         sl-MappedQoS-FlowsToReleaseList, if included;     -   3> perform the sidelink DRB release or modification procedure,         according to sub-clause 5.8.9.1.4 and 5.8.9.1.5.     -   1> if the UE is unable to comply with (part of) the         configuration included in the RRCReconfigurationSidelink (i.e.         sidelink RRC reconfiguration failure):     -   2> continue using the configuration used prior to the reception         of the RRCReconfigurationSidelink message;     -   2> set the content of the RRCReconfigurationFailureSidelink         message;     -   3> submit the RRCReconfigurationFailureSidelink message to lower         layers for transmission;     -   1> else:     -   2> set the content of the RRCReconfigurationCompleteSidelink         message;     -   3> submit the RRCReconfigurationCompleteSidelink message to         lower layers for transmission;     -   NOTE 1: When the same logical channel is configured with         different RLC mode by another UE, the UE handles the case as         sidelink RRC reconfiguration failure.         [ . . . ]

5.8.9.3 Sidelink Radio Link Failure Related Actions

The UE shall:

-   -   1> upon indication from sidelink RLC entity that the maximum         number of retransmissions for a specific destination has been         reached; or     -   1> upon T400 expiry:     -   2> consider sidelink radio link failure to be detected for this         destination;     -   2> release the DRBs of this destination, in according to         sub-clause 5.8.9.1.4;     -   2> release the SRBs of this destination, in according to         sub-clause 5.8.9.1.7;     -   2> discard the NR sidelink communication related configuration         of this destination;     -   2> consider the PC5-RRC connection is released for the         destination;     -   2> indicate the release of the PC5-RRC connection to the upper         layers for this destination (i.e. PC5 is unavailable);     -   2> if UE is in RRC_CONNECTED:     -   3> perform the sidelink UE information for NR sidelink         communication procedure, as specified in 5.8.3.3 or sub-clause         5.10.X in TS 36.331 [10];

3GPP TR 23.752 introduces the issue on support of UE-to-UE Relay and related solutions for a new release (i.e. Release 17) as follows:

5.4 Key Issue #4: Support of UE-to-UE Relay 5.4.1 General Description

This key issue intends to support for UE-to-UE Relay, including support for in coverage and out of coverage operation. At least the following aspects need to be considered in potential solutions:

-   -   How to (re)-select a UE-to-UE Relay UE in proximity?     -   Whether and how for the network can control the UE-to-UE Relay         operation, at least including how to:         -   Authorize the UE-to-UE Relay, e.g. authorize a UE as             UE-to-UE Relay?         -   Provide the visibility of source/target UE and the UE-to-UE             Relay to the network for the purpose of, e.g. charging?     -   How to establish the connection between the source UE and the         target UEs via UE-to-UE Relay?     -   How to provide end-to-end QoS framework to satisfy the QoS         requirements (such as data rate, reliability, latency)?     -   How to enhance the system architecture to provide the security         protection for relayed connection?     -   How to provide a mechanism for path changing in case of e.g.         UE-to-UE Relay changes?     -   NOTE 1: For the involvement of NG-RAN, coordination with RAN WGs         is needed.     -   NOTE 2: For security aspects, coordination with SA3 is needed.         [ . . . ]         6.8 Solution #8: UE-to-UE Relay Selection without Relay         Discovery

6.8.1 Description

This proposal aims to ensure the relay discovery between the source and the target UE shall not be dependent on how the relay forward traffic between the source and the target UE, e.g. L2 or L3 relaying. This solution relies on the concept that the UE-to-UE discovery and selection can be integrated into the unicast link establishment procedure as described in clause 6.3.3 of TS 23.287 [5]. A new field is proposed to be added in the direct communication request to indicate whether relays can be used in the communication. The field can be called relay_indication. When a UE wants to broadcast a direct communication request, it indicates in the message whether a UE-to-UE relay could be used. For Release 17, it is assumed that the value of the indication is restricted to single hop. When a UE-to-UE relay receives a direct communication request with the relay_indication set, then it shall decide whether to forward the request (i.e. broadcast this request in its proximity), according to e.g. the QoS requirements in the request, the current traffic load of the relay, the radio conditions between the source UE and the relay UE, or some other policies (e.g. it only serves some specific UEs or services). It may be the situation where multiple UE-to-UE relays can be used to reach the target UE or the target UE may also directly receive the direct communication request from the source UE. The target UE may choose which one to reply according to e.g. signal strength, local policy (e.g. traffic load of the UE-to-UE relays) or operator policies (e.g. always prefer direct communication or only use some specific UE-to-UE relays). The source UE may receive the direct communication accept message from multiple UE-to-UE relays and also from the target UE directly, the source UE chooses the communication path according to e.g. signal strength, local policy (e.g. traffic load of the UE-to-UE relays) or operator policies (e.g. always prefer direct communication or only use some specific UE-to-UE relays).

6.8.2 Procedures FIG. 6.8.2-1 of 3GPP TR 23.752 V0.3.0, Entitled “5G ProSe UE-to-UE Relay Selection”, is Reproduced as FIG. 15

FIG. 6.8.2-1 illustrates the procedure of the proposed method.

-   -   0. UEs are authorized to use the service provided by the         UE-to-UE relays. UE-to-UE relays are authorized to provide         service of relaying traffic among UEs. The authorization and the         parameter provisioning can use solutions for KI #8.     -   1. UE-1 wants to establish unicast communication with UE-2 and         the communication can be either through direct link with UE-2 or         via a UE-to-UE relay. Then UE-1 broadcasts directly         communication request with relay_indication=1. The request will         be received by relay-1, relay-2. The request may also be         received by UE-2 if it is in the proximity of UE-1.     -   2. Relay-1 and relay-2 decide to forward the request. They         broadcast the message in their proximity with         relay_indication=0. If a relay receives this message, it will         just drop it.     -   3. UE-2 receives the requests from relay-1 and relay-2.     -   4. UE-2 chooses relay-1 and replies with request accept. If UE-2         directly receives the direct communication request from UE-1, it         may choose to setup a direct communication link by sending the         request accept directly to UE-1. The response message includes         indication on the type of communication link being established         (e.g. via relay or direct).     -   5. UE-1 receives the request accept from relay-1. UE-1 chooses         path according to e.g. policies (e.g. always choose direct path         if it is possible), signal strength, etc. If UE-1 receives         request accept directly from UE-2, it may choose to setup a         direct L2 link as described in clause 6.3.3 of TS 23.287 [5],         then step 6 is skipped.     -   6. UE-1 and UE-2 setup communication link through chosen         UE-to-UE relay. The link setup information may vary depending on         the type of relay, e.g. L2 or L3 relaying.     -   NOTE 1: In order to make a relay or path selection, the source         UE can setup a timer after sending out the direct communication         request for collecting the corresponding request accept messages         before making a decision. Similarly, the target UE can also         setup a timer after receiving the first copy of the direct         communication request for collecting multiple copies of the         request from different paths before making a decision.     -   NOTE 2: In the first time when a UE receives a message from a         UE-to-UE relay, the UE needs to verify if the relay is         authorized be a UE-to-UE relay. The verification details and the         how to secure the communication between two UEs through a         UE-to-UE relay is to be defined by SA WG3.

6.8.3 Impacts on Existing Nodes and Functionality

UE impacts to support new Relay related functions.

6.9 Solution #9: Connection Establishment Via UE-to-UE Layer-2 Relay 6.9.1 Description

Using the solution described in this clause, a UE-to-UE Relay enables the discovery of a source UE by a target UE. A UE-to-UE Relay is authorized to relay messages between two UEs over the PC5 interface via authorization and provisioning, as defined in clause 6.Y Solution for Key Issue #4: UE-to-UE Relay Authorization and Provisioning. The source UE announces its supported applications or discovers a target UE using a known discovery mechanism, e.g. using user-oriented or service-oriented methods as defined in TS 23.287 [5]. The UE-to-UE Relay listens for ProSe applications advertisements (e.g. Direct Discovery or Direct Communication Request messages) from surrounding UEs and if a broadcasted application matches one of the applications from its provisioned relay policy/parameters, the UE-to-UE Relay advertises it as a relayed application by adding a relay indication to the message. A target UE discovers a source UE via a UE-to-UE Relay. The target UE receives a broadcast Direct Communication Request message with a relay indication. A secured “extended” PC5 link is set up between the source UE and the target UE via the UE-to-UE Relay. The source/target UEs do not know their respective peer UE's L2 IDs. Source/Target UEs send messages to the UE-to-UE Relay and receive messages through the UE-to-UE Relay. However, the security association and the PC5 unicast link are established directly between the source UE and target UE. The UE-to-UE Relay forwards the messages in opaque mode, without the ability to read, modify their content or replay them. The source/target UEs detect that the communication is going through a UE-to-UE Relay upon detecting a relay indication included in the received messages. The UE-to-UE Relay assigns itself two Relay-L2 IDs when a unicast link is established between two peer UEs via the UE-to-UE Relay. The first Relay-L2 ID is used when forwarding a message to the target UE. The second Relay-L2 ID is used when forwarding a message to the source UE. The UE-to-UE Relay maintains a mapping table containing the mapping of peer UEs L2 IDs and the corresponding Relay-L2 IDs that have been self-assigned. When receiving a message, the UE-to-UE Relay uses its mappings table to find the source and destination IDs to be used to forward the message to the target UE. The UE-to-UE Relay uses the Relay-L2 ID specified in the destination field to find the related UE and uses the UE's L2 ID specified in the source field to find the related Relay-L2 ID. It then updates the source and destination fields of the received message with its corresponding UE's L2 ID and Relay-L2 ID before forwarding the message.

-   -   NOTE: Additional security-related parameters and procedures may         be needed for the protection of relay related messages. Their         definitions need to be coordinated with SA WG3.

6.9.2 Procedures

The two methods defined in TS 23.287 [5], i.e. service-oriented and user-oriented are supported using the procedure described in this clause. FIG. 6.9.2-1 shows the peer discovery and unicast link establishment over PC5 reference point via a UE-to-UE Relay.

FIG. 6.9.2-1 of 3GPP TR 23.752 V0.3.0, Entitled “Connection Establishment Procedure Via a UE-to-UE Relay”, is Reproduced as FIG. 16

-   -   0. UE-to-UE Relay registers with the network and specifies its         UE-to-UE Relay capabilities. UE-to-UE Relay is provisioned from         the network with relay policy parameters and with a unique Relay         identifier (RID).     -   1. The target UEs (i.e. UE2, UE3 and UE4) determine the         destination Layer-2 ID for signalling reception for PC5 unicast         link establishment as specified in TS 23.287 [5] clause 5.6.1.4.         The destination Layer-2 ID is configured with the target UEs as         specified in TS 23.287 [5] clause 5.1.2.1.     -   2. On the source UE (i.e. UE1), the application layer provides         information to the ProSe layer for PC5 unicast communication         (e.g. broadcast Layer-2 ID, ProSe Application ID, UE's         Application Layer ID, target UE's Application Layer ID, relay         applicable indication), as specified in TS 23.287 [5] clause         6.3.3.1.     -   3. ProSe layer triggers the peer UE discovery mechanism by         sending a broadcast Direct Communication Request message. The         message is sent using the source Layer-2 ID and broadcast         Layer-2 ID as destination, and includes other parameters related         to the application offered, as specified in TS 23.287 [5] clause         6.3.3.1.     -   4. The UE-to-UE Relay receives the broadcast Direct         Communication Request message and verifies if it's configured to         relay this application, i.e. it compares the announce ProSe         Application ID with its provisioned relay policy/parameters and,         if it matches, the UE-to-UE Relay assigns itself a Relay-Layer-2         ID (e.g. R-L2 ID-a) for UE1 (i.e. related to UE1's L2 ID).         These 2 IDs (UE1's Layer-2 ID and Relay-Layer-2 ID-a) are saved         in a local mapping table. The UE-to-UE Relay overrides the         source field of the message with its R-L2 ID-a and adds its         unique relay identifier (RID) as a relay indication. This relay         indication is added by the UE-to-UE Relay only on broadcast         messages since these messages are sent in clear text (i.e.         without any encryption or integrity protection) thus may be         modified. The UE-to-UE Relay proceeds in forwarding the         broadcast Direct Communication Request message received from the         source UE.     -   5. Target UE3 is interested in the announced application thus,         it triggers the authentication and security establishment with         UE1, via the UE-to-UE Relay. UE3 keeps track of the Relay's         identifiers, i.e. R-L2 ID-a and RID. UE3 sends the RID in a         security protected message during the authentication and         security establishment to inform UE1 that the communication is         traversing the UE-to-UE Relay identified by RID.         UE-to-UE Relay receives the message from UE3 and uses the R-L2         ID-a specified in the destination field to find the related UE         (i.e. UE1 in this case) in its mapping table.         UE-to-UE Relay assigns itself a new Layer-2 ID (e.g. R-L2 ID-b)         for UE3 and stores the mapping between UE3's L2 ID and R-L2         ID-b.         UE-to-UE Relay sets the source field of the message to R-L2 ID-b         and sets the destination field to UE1's Layer-2 ID (i.e. L2 ID1)         retrieved from the mapping entry. UE-to-UE Relay sends the         message to UE1.         UE1 receives the authentication message and keeps track of R-L2         ID-b and RID. R-L2 ID-b is used as the destination on subsequent         messages destined to UE3 and sent via the UE-to-UE Relay.         Authentication and security establishment messages are exchanged         between UE1 and UE3 via the UE-to-UE Relay. UE-to-UE Relay         changes the source/destination Layer-2 IDs based on the         information saved in its local mapping table.     -   Editor's note: The Details of the authentication and security         procedure will be investigated by SA WG3 group.     -   6. Once the security is established, UE3 completes the unicast         link establishment by sending a Direct Communication Accept         message.     -   7. UE-to-UE Relay receives the message and sets the source field         of the message to the R-L2 ID-b as found in the mapping entry         and sets the destination field to the UE1's L2 ID also from the         mapping entry. UE-to-UE Relay sends the modified message to UE1.     -   8. An “extended” unicast link is established between UE1 and         UE3, via the UE-to-UE Relay. The extended link is secured end to         end, i.e. a security association has been created between UE1         and UE3. Confidentiality and/or integrity/replay protected         messages (i.e. data or PC5-S) may be exchanged between UE1 and         UE3. The UE-to-UE Relay is not involved in the security         association thus it cannot read nor modify the secured portion         of the message (which excludes the source and destination         fields).     -   Editor's note: The details of protocol stack and PC5 link         establishment is FFS and need to be co-ordinated and confirmed         by RAN WG2 group.

6.9.3 Impacts on Services, Entities and Interfaces

The solution has impacts in the following entities:

UE:

-   -   Needs to support procedures for ProSe 5G UE-to-UE Relay and         communications via a ProSe 5G UE-to-UE Relay.

6.10 Solution #10: ProSe 5G Layer-3 UE-to-UE Relay Based on IP Routing 6.10.1 Description

In this solution, the ProSe 5G UE-to-UE Relay operations is supported with the following principles:

-   -   Authorization and configuration:         -   Only the UE authorized by the service authorization             configuration can act as a ProSe 5G UE-to-UE Relay. These             UEs will be configured according to the service             authorization and provisioning mechanism defined in TS             23.287 [5] to operate in the UE-to-UE Relay mode.     -   ProSe 5G UE-to-UE Relay discovery:         -   The ProSe 5G UE-to-UE Relay sends out a Relay Discovery             message periodically, announcing its availability for             serving other UEs in the area.         -   The ProSe 5G UE-to-UE Relay also supports the query and             response mode for discovery. The ProSe 5G UE-to-UE Relay             listens on a configured Layer-2 ID for the query, and would             respond with its address and corresponding information to             enable to other UE to establish a unicast connection with             it. This process is similar to the unicast L2 link             establishment procedure as defined in TS 23.287 [5] clause             6.3.3.1.     -   NOTE 1: The Layer-2 ID used for the discovery can be specific         for UE-to-UE Relay discovery, or shared with other discoveries,         e.g. UE-to-Network Relay discovery.         -   ProSe 5G UE-to-UE Relay operation:             -   Any UE that wants to make use of the ProSe 5G UE-to-UE                 Relay needs to establish a unicast L2 link with the                 UE-to-UE Relay, with IP configuration. The ProSe 5G                 UE-to-UE Relay allocates IP address/prefix to the other                 UEs.             -   As part of the unicast L2 link establishment procedure,                 the ProSe 5G UE-to-UE Relay stores an association of the                 User Info of the peer UE of the unicast link and the IP                 address/prefix allocated to the UE into its DNS entries.                 The ProSe 5G UE-to-UE Relay acts as a DNS server to                 other UEs.             -   When a (source) UE needs to communicate with another                 (target) UE or needs to discover a ProSe service via the                 ProSe 5G UE-to-UE Relay, it sends a DNS query for the                 target UE (based on Target User Info) or for the ProSe                 Service to the ProSe 5G UE-to-UE Relay over the unicast                 link, which will return the IP address/prefix of the                 target UE or the ProSe Service.             -   The source UE sends the IP data or non-IP data                 encapsulated in IP to the target UE via the unicast L2                 link to UE-to-UE Relay that returned the IP                 address/prefix of the target UE. The ProSe 5G UE-to-UE                 Relay acts as an IP router, and forwards the packets to                 the corresponding unicast L2 link towards the target UE.                 Each of the unicast L2 link is treated as an IP                 interface.             -   If there are multiple ProSe 5G UE-to-UE Relays in the                 proximity, UE can choose either one or more ProSe 5G                 UE-to-UE Relays to establish the unicast L2 link based                 on UE implementation. For example, the UE sends a DNS                 query on each of the unicast L2 link to the ProSe 5G                 UE-to-UE Relays. Then, the source UE may choose to use                 the first ProSe 5G UE-to-UE Relay that returns a                 positive DNS query for the target UE.     -   NOTE 2: The selection of the UE-to-UE Relay may be based on         local configured rules on the UE, or based on other discovery         solutions, e.g. “Stateful UE-to-UE Relay” described in clause         6.11.         -   QoS handling:             -   When the source UE establishes the unicast L2 link with                 the ProSe 5G UE-to-UE Relay, it can establish                 corresponding PC5 QoS Flows according to procedure                 defined in clause 6.3.3.1 of TS 23.287 [5]. It can also                 modify the PC5 QoS Flows at any time using procedure                 defined in clause 6.3.3.4 of TS 23.287 [5].             -   Correspondingly, the ProSe 5G UE-to-UE Relay can also                 establish and modify the PC5 QoS Flows using the                 above-mentioned procedures over the unicast L2 Link with                 the target UE for the forwarding of source UE's traffic.         -   Security handling:             -   source UE and target UE can establish bearer level                 security with the UE-to-UE Relay for the unicast L2                 Link, using procedures defined in TS 23.287 [5].             -   If end-to-end security protection is required between                 source UE and target UE, IPSec can be used.     -   NOTE 3: The security protection of the traffic of source UE and         target UE will be specified by SA WG3.         -   Charging Support:             -   ProSe 5G UE-to-UE Relay can follow the charging solution                 defined in TS 32.277 [13] to report the source and                 target UEs and corresponding traffic to the charging                 function.

6.10.2 Procedures [FIG. 6.10.2-1 of 3GPP TR 23.752 V0.3.0, Entitled “5G ProSe UE-to-UE Relay Operation”, is Reproduced as FIG. 17]

FIG. 6.10.2-1 provides an example operation for the 5G ProSe UE-to-UE Relay operation based on standard IP operation.

6.10.3 Impacts on Services, Entities and Interfaces

There is no impact to NG-RAN, as the solution is using the existing features supported in Rel-16 NR V2X design. UEs operates with existing IP operation, and the ProSe 5G UE-to-UE Relay supports the IP router function (for address allocation and traffic forwarding) and the functionality of a DNS server. [ . . . ]

Key issue #4 in 3GPP TR 23.752 describes support of UE-to-UE Relay in the next release (i.e. Release 17), which means a relay may be used to support data communication between two UEs in case these two UEs cannot communicate with each other directly. It is supposed that a UE-to-UE Relay needs to establish one PC5 unicast link with each of a Source UE and a Target UE such that the integrated PC5 unicast link between the Source UE and the Target UE can support the concerned ProSe service as illustrated in FIG. 18, which shows an example of Integrated PC5 unicast link via a User Equipment-to-User Equipment (UE-to-UE) Relay according to one embodiment.

PC5 unicast link with each of a Source UE and a Target UE such that the integrated PC5 unicast link between the Source UE and the Target UE can support the concerned ProSe service as illustrated in FIG. 18.

According to 3GPP TS 38.331, the UE shall release all Sidelink (SL) Data Radio Bearers (DRBs) established for a PC5 unicast link established with a peer UE if sidelink radio link failure is detected. In the scenario of a UE-to-UE relaying, if sidelink radio link failure is detected on one of the two PC5 unicast links established with the Source UE and the Target UE, the data from the other PC5 unicast link would accumulate in buffer of the UE-to-UE Relay, which would consume the buffer space of the UE-to-UE Relay unnecessarily because the data cannot be relayed (or forwarded) to the destination via the failed PC5 unicast link. Therefore, it would be beneficial for the UE-to-UE Relay to initiate a Layer-2 link release procedure to release the PC5 unicast link with the other UE when detecting a sidelink radio link failure on the PC5 unicast link with one UE. FIG. 19 illustrates an example of this solution.

For similar reasons, it would also be beneficial for the UE-to-UE Relay to initiate a Layer-2 link release procedure to release the PC5 unicast link with the other UE in response to reception of a Disconnect Request message from one UE to disconnect or release the PC5 unicast link with one UE. In other words, the UE-to-UE Relay may initiate a Layer-2 link release procedure to release the PC5 unicast link with the other UE when (or if) receiving a Disconnect Request message from one UE or when (or if) a Disconnect Response message is transmitted in response to reception of the Disconnect Request message.

FIG. 20 is a flow chart 2000 according to one exemplary embodiment from the perspective of a UE-to-UE Relay. In step 2005, the UE-to-UE Relay establishes a first PC5 unicast link with a first UE and establishes a second PC5 unicast link with a second UE. In step 2010, the UE-to-UE Relay initiates a Layer-2 link release procedure to release the second PC5 unicast link if a sidelink radio link failure is detected on the first PC5 unicast link.

In one embodiment, the UE-to-UE Relay could transmit a first PC5-S message to the second UE when initiating the Layer-2 link release procedure. The first PC5-S message could be a Disconnect Request message.

In one embodiment, the UE-to-UE Relay could receive a second PC5-S message from the second UE. The second PC5-S message could be a Disconnect Response message.

In one embodiment, the sidelink radio link failure on the first PC5 unicast link could be detected upon indication from a sidelink RLC entity that the maximum number of retransmissions for a specific destination of the first PC5 unicast link has been reached. The sidelink radio link failure on the first PC5 unicast link could also be detected upon expiry of a timer which is started when a RRCReconfigurationSidelink message is submitted to lower layers for transmission.

In one embodiment, the first UE and the second UE communicate with each other via the UE-to-UE Relay

Referring back to FIGS. 3 and 4, in one exemplary embodiment of a UE-to-UE Relay, the UE-to-UE Relay 300 includes a program code 312 stored in the memory 310. The CPU 308 could execute program code 312 to enable the UE-to-UE Relay (i) to establish a first PC5 unicast link with a first UE and establishes a second PC5 unicast link with a second UE, and (ii) to initiate a Layer-2 link release procedure to release the second PC5 unicast link if a sidelink radio link failure is detected on the first PC5 unicast link. Furthermore, the CPU 308 can execute the program code 312 to perform all of the above-described actions and steps or others described herein.

Various aspects of the disclosure have been described above. It should be apparent that the teachings herein could be embodied in a wide variety of forms and that any specific structure, function, or both being disclosed herein is merely representative. Based on the teachings herein one skilled in the art should appreciate that an aspect disclosed herein could be implemented independently of any other aspects and that two or more of these aspects could be combined in various ways. For example, an apparatus could be implemented or a method could be practiced using any number of the aspects set forth herein. In addition, such an apparatus could be implemented or such a method could be practiced using other structure, functionality, or structure and functionality in addition to or other than one or more of the aspects set forth herein. As an example of some of the above concepts, in some aspects concurrent channels could be established based on pulse repetition frequencies. In some aspects concurrent channels could be established based on pulse position or offsets. In some aspects concurrent channels could be established based on time hopping sequences. In some aspects concurrent channels could be established based on pulse repetition frequencies, pulse positions or offsets, and time hopping sequences.

Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Those of skill would further appreciate that the various illustrative logical blocks, modules, processors, means, circuits, and algorithm steps described in connection with the aspects disclosed herein may be implemented as electronic hardware (e.g., a digital implementation, an analog implementation, or a combination of the two, which may be designed using source coding or some other technique), various forms of program or design code incorporating instructions (which may be referred to herein, for convenience, as “software” or a “software module”), or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

In addition, the various illustrative logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented within or performed by an integrated circuit (“IC”), an access terminal, or an access point. The IC may comprise a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, electrical components, optical components, mechanical components, or any combination thereof designed to perform the functions described herein, and may execute codes or instructions that reside within the IC, outside of the IC, or both. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

It is understood that any specific order or hierarchy of steps in any disclosed process is an example of a sample approach. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged while remaining within the scope of the present disclosure. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented.

The steps of a method or algorithm described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module (e.g., including executable instructions and related data) and other data may reside in a data memory such as RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of computer-readable storage medium known in the art. A sample storage medium may be coupled to a machine such as, for example, a computer/processor (which may be referred to herein, for convenience, as a “processor”) such the processor can read information (e.g., code) from and write information to the storage medium. A sample storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in user equipment. In the alternative, the processor and the storage medium may reside as discrete components in user equipment. Moreover, in some aspects any suitable computer-program product may comprise a computer-readable medium comprising codes relating to one or more of the aspects of the disclosure. In some aspects a computer program product may comprise packaging materials.

While the invention has been described in connection with various aspects, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptation of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within the known and customary practice within the art to which the invention pertains. 

1. A method for a User Equipment-to-User Equipment (UE-to-UE) Relay, comprising: the UE-to-UE Relay establishes a first PC5 unicast link with a first UE and establishes a second PC5 unicast link with a second UE; and the UE-to-UE Relay initiates a Layer-2 link release procedure to release the second PC5 unicast link if a sidelink radio link failure is detected on the first PC5 unicast link.
 2. The method of claim 1, wherein the UE-to-UE Relay transmits a first PC5-S message to the second UE when initiating the Layer-2 link release procedure.
 3. The method of claim 2, wherein the first PC5-S message is a Disconnect Request message.
 4. The method of claim 1, wherein the UE-to-UE Relay receives a second PC5-S message from the second UE.
 5. The method of claim 4, wherein the second PC5-S message is a Disconnect Response message.
 6. The method of claim 1, wherein the sidelink radio link failure on the first PC5 unicast link is detected upon indication from a sidelink RLC entity that the maximum number of retransmissions for a specific destination of the first PC5 unicast link has been reached.
 7. The method of claim 1, wherein the sidelink radio link failure on the first PC5 unicast link is detected upon expiry of a timer which is started when a RRCReconfigurationSidelink message is submitted to lower layers for transmission.
 8. The method of claim 1, wherein the first UE and the second UE communicate with each other via the UE-to-UE Relay.
 9. A User Equipment-to-User Equipment (UE-to-UE) Relay, comprising: a control circuit; a processor installed in the control circuit; and a memory installed in the control circuit and operatively coupled to the processor; wherein the processor is configured to execute a program code stored in the memory to: establish a first PC5 unicast link with a first UE and establishes a second PC5 unicast link with a second UE; and initiate a Layer-2 link release procedure to release the second PC5 unicast link if a sidelink radio link failure is detected on the first PC5 unicast link.
 10. The UE-to-UE Relay of claim 9, wherein the processor is configured to execute the program code stored in the memory to: transmit a first PC5-S message to the second UE when initiating the Layer-2 link release procedure.
 11. The UE-to-UE Relay of claim 10, wherein the first PC5-S message is a Disconnect Request message.
 12. The UE-to-UE Relay of claim 9, wherein the processor is configured to execute the program code stored in the memory to: receive a second PC5-S message from the second UE.
 13. The UE-to-UE Relay of claim 12, wherein the second PC5-S message is a Disconnect Response message.
 14. The UE-to-UE Relay of claim 9, wherein the sidelink radio link failure on the first PC5 unicast link is detected upon indication from a sidelink RLC entity that the maximum number of retransmissions for a specific destination of the first PC5 unicast link has been reached.
 15. The UE-to-UE Relay of claim 9, wherein the sidelink radio link failure on the first PC5 unicast link is detected upon expiry of a timer which is started when a RRCReconfigurationSidelink message is submitted to lower layers for transmission.
 16. The UE-to-UE Relay of claim 9, wherein the first UE and the second UE communicate with each other via the UE-to-UE Relay. 